The "Donate Anonymously" field on the My Contact Section is currently available for peer-to-peer forms.  For Peer-to-Peer, the Sidebar Widget that shows previous donations and names will NOT show the name if this checkbox is selected.

There is a Fieldset called- "checkouttransactiondetails" on the Payment Txn object. You can add the Donate Anonymously field on to that Fieldset and then the field will appear on the Transaction Details Section of the form.

When adding the field here, it is used to note the donate anonymous request only and has no further function.

Please make sure to do Refresh the form builder after adding the field to the field set.